"Drop the hammer on them."
That's the easy part. The hard part is dealing with what happens after the hammer has been dropped.
Someone once said that the definition of a bad policy is one that leads to a place where you have nothing but bad options. I believe everyone (not just the Greeks) thought back in 2000 it woudl be good policy to bring Greece into the Eurozone. But now we've now reached the point where otherwise rational people are talking about "dropping the hammer", as if having an incipient failed state in Europe is a small price to pay for 600 euro in your pocket. The frustration is understandable, but the the satisfaction of dropping the hammer on Greece would be short-lived -- possibly on the order of weeks depending on the scale of financial disruption.
The unhappy truth is that bad policy choices fifteen years ago means all the options available today lead to long-lived, complicated, and expensive consequences.